Biological/Biomedical Accelerator Mass Spectrometry Targets. 1. Optimizing the CO₂ Reduction Step Using Zinc Dust

Abstract

Biological and biomedical applications of accelerator mass spectrometry (AMS) use isotope ratio mass spectrometry to quantify minute amounts of long-lived radioisotopes such as ¹⁴C. AMS target preparation involves first the oxidation of carbon (in sample of interest) to CO₂ and second the reduction of CO₂ to filamentous, fluffy, fuzzy, or firm graphite-like substances that coat a −400-mesh spherical iron powder (−400MSIP) catalyst. Until now, the quality of AMS targets has been variable; consequently, they often failed to produce robust ion currents that are required for reliable, accurate, precise, and high-throughput AMS for biological/biomedical applications. Therefore, we described our optimized method for reduction of CO₂ to high-quality uniform AMS targets whose morphology we visualized using scanning electron microscope pictures. Key features of our optimized method were to reduce CO₂ (from a sample of interest that provided 1 mg of C) using 100 ± 1.3 mg of Zn dust, 5 ± 0.4 mg of −400MSIP, and a reduction temperature of 500 °C for 3 h. The thermodynamics of our optimized method were more favorable for production of graphite-coated iron powders (GCIP) than those of previous methods. All AMS targets from our optimized method were of 100% GCIP, the graphitization yield exceeded 90%, and δ¹³C was −17.9 ± 0.3‰. The GCIP reliably produced strong ¹²C⁻ currents and accurate and precise <i>F</i>_m values. The observed <i>F</i>_m value for oxalic acid II NIST SRM deviated from its accepted <i>F</i>_m value of 1.3407 by only 0.0003 ± 0.0027 (mean ± SE, <i>n</i> = 32), limit of detection of ¹⁴C was 0.04 amol, and limit of quantification was 0.07 amol, and a skilled analyst can prepare as many as 270 AMS targets per day. More information on the physical (hardness/color), morphological (SEMs), and structural (FT-IR, Raman, XRD spectra) characteristics of our AMS targets that determine accurate, precise, and high-hroughput AMS measurement are in the companion paper